1. Field of the Invention
The invention relates to a continuous process for the preparation of aryl carbonates from carbonates containing at least one aliphatic ester group on the one hand and phenols or alkyl aryl carbonates on the other hand by catalyzed transesterification in a column-type reactor with multiple recycling of the reaction products into the same reactor and intermediate storage of the product streams in suitable vessels.
2. Description of the Related Art
The preparation of aromatic and aliphatic-aromatic carbonates by transesterification starting from aliphatic carbonates and phenols is known in principle. This is an equilibrium reaction, the position of the equilibrium being almost completely displaced in the direction of the aliphatically substituted carbonates. Therefore, it is relatively easy to prepare aliphatic carbonates from aromatic carbonates and alcohols. However, in order to carry out the reaction in the reverse direction towards aromatic carbonates, it is necessary effectively to displace the highly unfavourable equilibrium, where not only highly active catalysts but also a favourable procedure have to be used.
A multiplicity of effective catalysts, such as alkali metal hydroxides, Lewis acid catalysts selected from the group comprising the metal halides (German Offenlegungsschrift 25 28 412 and 25 52 907), organotin compounds (EP 879, EP 880, German Offenlegungsschrift 34 45 552, EP 338 760), lead compounds (JP-57/176 932) and Lewis acid/proton acid catalysts (German Offenlegungsschrift 34 45 553) have been recommended for the transesterification of aliphatic carbonates by phenols. In the known processes the transesterification is carried out in a batch reactor under atmospheric pressure or super-atmospheric pressure, if necessary, using an additional separation column. Even using the most active catalysts, reaction times of many hours are required even to achieve mean conversions of only approximately 50% of phenol. Thus, in the batchwise transesterification of phenol with diethyl carbonate at 180.degree. C. using various organotin compounds, as described in German Offenlegungsschrift 34 45 552, yields of diphenyl carbonate of an order of magnitude of above 20% are only achieved after a reaction time of approximately 24 hours; in the batchwise transesterification of phenol and dimethyl carbonate with the aid of organotin catalysts, as described in EP 879, the phenol conversion after 30 hours is 34% of the theoretical value.
This means that on account of the unfavourable thermodynamic conditions, the described transesterification reactions in tanks or pressure autoclaves, even with the use of highly active catalyst systems, can only be carried out highly disadvantageously in the sense of an industrial process, since very poor space-time yields and high residence times at high reaction temperatures are required, where, because of the incomplete transesterification, a high distillation effort must additionally be applied which requires further energy.
Such procedures are also particularly disadvantageous since, even using highly selective transesterification catalysts, at the high temperatures and long residence times of many hours, a significant amount of side-reactions occurs, for example ether formation and the elimination of carbon dioxide.
An attempt was therefore made to displace the reaction equilibrium as rapidly as possible in the direction of the desired products by adsorption of the alcohol forming in the transesterification to molecular sieves (German Offenlegungsschrift 33 08 921). It is shown from the description of this procedure that a large amount of molecular sieve is required for the adsorption of the reaction alcohol, which far exceeds the amount of alcohol being liberated. In addition, the molecular sieves used must be regenerated even after a short time and the conversion rate to the alkylaryl carbonate intermediates is relatively low. This method therefore also appears not to be advantageously applicable industrially.
It is known to carry out equilibrium reactions, in particular esterifications and transesterifications, in columns, to accelerate them in this manner and to displace them in the direction of the desired products (Chem.-Ing.-Techn. 49, 151 (1977); German Offenlegungsschrift 38 09 417; Chem.-Ing.-Techn. 62, 226 (1990); Ullmanns Encyclopadie der techn. Chemie [Ullmanns Enyclopaedia of Industrial Chemistry], 4th edition, volume 3, pp. 375 ff. (1973)). A continuous transesterification process described in WO 91/09832=EP 0 461 274 for the preparation of aromatic carbonates in which the reaction is carried out in a multiple step manner in sequentially connected columns is an optimized development of this reaction principle. In the columns described, phenols are reacted with dialkyl carbonates, the lower boiling reaction products, that is aliphatic alcohols, together with unreacted dialkyl carbonates being withdrawn via the head of the column and the higher boiling reaction products, that is alkyl aryl carbonates and, possibly, diaryl carbonates being withdrawn at the foot of the column. In a further downstream column, the alkyl aryl carbonates already formed are reacted to form the desired diaryl carbonate end products. The dialkyl carbonates formed as coupling products and, possibly, alcohols and the still unreacted phenols are withdrawn at the top end of the column and partly or completely recycled to the first column. However, as can be deduced from the example embodiments and the process variants described, the conversions of the phenols and dialkyl carbonates in the first transesterification column, even under favourable conditions such as high temperatures and pressures and large excesses of dialkyl carbonates of 100 to 300%, are restricted to low values, that is, even in favourable cases as in Example 10 of WO 91/09832, the bottom contains only approximately 15% by weight of transesterification products, essentially methyl phenyl carbonate.
This means that in the subsequent second column, only a small part of the starting material stream, that is the alkyl phenyl carbonate already formed, can be converted to the diaryl carbonate end product with the disadvantageous consequence that the overwhelming majority of the remainder, which is essentially composed of phenol, must be removed via the head by distillation and returned, following condensation, to the first column.
A disproportionately high distillation effort and expenditure of energy must be applied for this. The columns to be used must have large volumes for a given amount of product per unit of time and require high investment costs; in particular, distilling off the large amounts of phenol, dialkyl carbonate and, possibly, alcohol, which is preferably carried out in vacuo, large amounts of gas being formed, requires a very large column which is thus expensive and difficult to operate. Moreover, the control of a plurality of continuously operated and sequentially connected columns, whose product streams are each independent of the other, is complex and difficult.
However, the improvement, albeit unsatisfactory, achieved according to WO 91/09832 is not surprising, since it is generally known that transesterifications in columns frequently proceed more rapidly, which is just what is observed in the present case. However, the conversions obtained are very low and the assumption is apparently made that, at the unfavourable equilibrium position (K.perspectiveto.10.sup.-3), even under optimal conditions, that is high temperatures and pressures, they virtually cannot be further increased, that is only with very high expenditure. Such an unfavourable equilibrium position means that at equilibrium only approximately 2 to 3% by weight of the product are present and the conversion can only be further increased if a product component, here the reaction alcohol, is removed and the reaction system can reestablish equilibrium. This process would have to be repeated several times. When equilibrium is established slowly, as in the present case, very long, multiple-tray columns would have to be used which would be able to be operated only at low load and low space-time yield. Such conditions were obviously considered in WO 91/09832 as unrealizable.
The aim of an improvement of the transesterification process must therefore be to realize greater phenol conversions and lower residual contents of phenol in the bottom product than hitherto using a suitable reaction apparatus under suitable conditions in a continuous, as simple as possible procedure.